Hydrodynamic Solvation of Poly(amido amine) Dendrimer Monolayers on Silica

Adsorption of poly(amido amine) dendrimers (G8 generation) on silica substrates was studied by atomic force microscopy (AFM), quartz crystal microbalance (QCM), and optical waveguide lightmode spectroscopy (OWLS). Initially, diffusion coefficient, hydrodynamic diameter, electrophoretic mobility, and stability of the dendrimers were determined as a function of pH and ionic strength for various electrolytes. Adsorption kinetics measurements performed by AFM allowed to determine the porosity of the dendrimers and their internal solvation by electrolytes. Subsequent kinetics measurements were carried out under flow conditions by applying QCM and OWLS techniques. The acquired experimental data were interpreted in terms of a hybrid random sequential adsorption approach. This furnished total solvation comprising internal and external contributions due to the hydrodynamic coupling effect. Two solvation functions were defined, which enabled to quantitatively analyze the QCM kinetics data. It is confirmed that the hydrodynamically coupled electrolyte amount in dendrimer layers is many times larger than the internally coupled electrolyte responsible for swelling. It is also shown that the hydrodynamically coupled electrolyte mass depends on the sensor oscillation frequency (overtone number) and the dendrimer coverage. This indicates that in the interpretation of dendrimer swelling phenomena, one should consider the dominant role of the solvent coupling due to hydrodynamic effects.